Immunising toddlers with mouth drops has reduced the number of polio cases from 350 000 children annually in the 1980s to fewer than 800 worldwide last year. Yet the virus is spreading again from Nigeria, where United Nations officials say a third of the world's cases are the result of a vaccine boycott.

Amid rising Muslim-Western tensions worldwide, Nigeria's Muslims are heeding allegations that the vaccine is an American plot to spread Aids or infertility.

Since October, three northern Nigerian states have banned door-to-door vaccinations until they are satisfied the vaccines do not contain harmful substances.

"Since September 11, the Muslim world is beginning to be suspicious of any move from the Western world," said Sule Ya'u Sule, spokesperson for the governor of Kano, one of the states where the vaccine is banned. "Our people have become really concerned about polio vaccine."

UN and Nigerian federal government officials stress the vaccines have repeatedly been proven safe. But detractors don't believe it, and meanwhile polio strains are spreading from northern Nigeria's trading center of Kano to at least seven nearby countries where the disease was previously eradicated, says the World Health Organisation's Bruce Aylward.

On Sunday, Nigeria sent a team of 12 scientists, government officials and Muslim leaders to South Africa, Indonesia and India to spend a week witnessing tests that would dispel the suspicions.

Muslims in Nigeria's arid north have become increasingly wary of vaccine initiatives since 1996, when families in Kano accused New York-based Pfizer of using an experimental meningitis drug on patients without fully informing them of the risks.

The company denied any wrongdoing and a US court dismissed a lawsuit by 20 disabled Nigerians alleged to have taken part in the study, but a US appeals court revived it late last year.

Zubairu Shaba, a former journalist who has lobbied the Nigerian government for compensation on behalf of the Pfizer patients' families, said he and others distrust the entire Western medical establishment.

"So many families won't go to hospitals again. They prefer to die," Shaba said. "We are suspicious of people who come to our doors with liquid for our children's mouths. We don't know who they are or what they want."

Not everyone agrees. "I've heard lots of people saying bad things about polio vaccine. I don't believe it," 22-year-old Habiba Nara said as a nurse at a clinic in northern Nigeria put vaccine drops in the mouth of Abubakar, her screaming 40-day-old baby boy.

But fears mounted last year after Datti Ahmed, a Kano physician who heads a prominent Muslim group, the Supreme Council for Shariah in Nigeria, said polio vaccines were "corrupted and tainted by evildoers from America and their Western allies".

Subsequent tests initiated by the federal government in Nigeria and South Africa proved conclusively the vaccines were free of all harmful substances, officials say.

Muslim groups rejected the results. Kano state officials insisted their own scientists tested the vaccines and found trace amounts of estrogen and progesterone, female sex hormones which the officials feared could cause infertility.

Jama'atu Nasril Islam, an influential Muslim group, said it sponsored its own tests in Britain and India and got similar results.

"We believe that modern-day Hitlers have deliberately adulterated the oral polio vaccines with anti-fertility drugs and contaminated with certain viruses which are known to cause HIV and Aids," Ahmed, in glasses and a threadbare gray robe, told The Associated Press on his front porch in Kano.

Aylward, the UN official, says any test results showing hormones are "false positives" arising from improper testing methods or the mixing of foreign materials during testing.

Even hormones at the levels alleged by critics would be of "absolutely of no health consequence" and amount to less than the amount found naturally in mothers' breast milk, Aylward said.

Officials in President Olusegun Obasanjo's government privately claim it's political - that Nigeria's main opposition party is hyping the controversy to avenge election victories by Obasanjo last year which international monitors said were tainted by fraud.

Sule, spokesperson for the opposition Kano state governor, admits politics plays a role.

"People in this part of the country believe they manipulated the vote, so they can manipulate anything," Sule said. These people will never trust the vaccine, no matter what assurances they get, he said. - Sapa-AP